This application generally relates to satellites, and more particularly, to systems and methods for tracking objects in space.
Characterizing objects in space to determine their status is important. This may include, for instance, detection and resolution of anomalies for spacecrafts, satellites, debris or other space-based objects. In addition, detecting new objects, verifying the position and velocity of existing objects, locating objects that have been maneuvered, and collecting imagery of objects serve a number of military, civil, and commercial needs in terms of avoiding collisions in space and understanding the operating status of satellites.
Space situational awareness (SSA) is knowing the location of objects orbiting the Earth. SSA techniques may include using ground-based observation, such as telescopes. In addition, conventional SSA systems may use a variety of ground-based radio frequency (RF) and optical sensors to provide a “fence” for timely detection and tracking of resident space objects (RSOs) at low-earth orbit (LEO), as well as debris in LEO and other orbits.
FIG. 1 depicts conventional LEO satellite system 100 for space situational awareness. Satellite 110 orbiting Earth 50 may be an upward looking LEO satellite, such as, for example, Midcourse Space Experiment (MSX) and Space-based Visible Sensing Systems (SBSS). In addition, satellite 110 may use visible (VIS) and possibly infrared (IR) sensors to provide timely access to geosynchronous (GEO) RSOs and other objects positioned along Geo belt 120. However, as shown, field of view (FOV) 130 of satellite 110 is quite limited.
FIG. 2 depicts another conventional LEO satellite system 200 for space situational awareness. Satellite 210 may include downward looking sensors, such as Raytheon's Space Tracking & Surveillance System (STSS) sensor, that look past the horizon of Earth 50 to scan GEO arc 220 and other high altitude objects. A problem with such a system is that much of the line-of-sight of each sensor may be blocked by Earth 50. As a result, access portions 230 are much smaller than no access portions 240.
The conventional satellite systems shown in FIGS. 1 and 2, therefore, are not optimal to provide timely access to satellites in either LEO or GEO orbits. In particular, there is a lack of angular diversity to any given GEO object such that, if an object is too close to the apparent position of the sun and/or the object is not illuminated from the vantage of one observer, this will be true of all observers.
Imaging of GEO objects, though, is typically performed using special ground assets and/or by flying sensors in co-orbits with selected GEO satellites. The vast majority of satellites are positioned in GEO orbit at approximately 35,800 km altitude from Earth. Ground based sensors can detect some objects at GEO altitudes, but this required telescope sizes are prohibitive to collect sufficiently high resolution imagery. In principle, ground-based radar systems can collect high-resolution imagery independent of the range to the target using inverse synthetic aperture techniques. However, the extremely small velocity of most satellites at GEO makes this difficult or impossible in practice. Similarly, it is prohibitive to search the large volume at GEO for small objects whose locations are not known a priori. Conventional collection of information from ground-based assets, however, becomes increasing difficult for satellites at this high altitude. Such observations may use of VIS and IR space based sensors, but may not be able to effectively track small objects in space. Also, ground-based observation must wait for a RSO to fly overhead. This may take many hours.
A VIS or IR sensor positioned in LEO orbit can access and detect many objects in LEO orbits within a few hours. But, it may take almost a day for proper viewing conditions for all objects to occur. In order to use a VIS sensor for observing a target against a sky and/or star-lit background, the target must be illuminated by the sun on the side of object facing the sensor, and the sun and the earth must be away from the sensor's optical axis. This event may not occur for 13 hours or more, from any given point in time. IR sensors do not require solar illumination, but may require cryogenic cooling and/or a larger aperture (as the IR signature of RSO's is relatively weak compared to the VIS illuminated signatures).
A LEO satellite sensor may have access to observe almost any object in other orbits, such as, medium earth orbit (MEO), highly elliptical orbit (HEO) or GEO, within a few hours period. However, small objects located in other orbits may be too faint to detect. For instance, objects in non-LEO orbits will be distant and will have faint signatures compared to objects in LEO orbits.
U.S. Patent Application Publication No. 2008/0081556, herein incorporated by reference in its entirety, discloses a system to rapidly image objects at GEO particularly using radar techniques. It discloses that N satellites could visit all objects in the GEO belt within 12/N hours. Thus, even small objects could be detected and tracked in a timely fashion. Satellite sensors in a retro-GEO orbit can detect small RSOs in GEO orbit and access all objects at GEO within 12 hours. These sensors, however, can see very few objects in LEO, as they must view them in a limited annulus near the Earth, and only when the object is not near a sunlit portion of the Earth.
The Unites States Air Force has discussed plans to deploy a constellation of up to four LEO satellites hosting electro-optic payloads to provide timely detection and tracking information on objects in all orbits. This satellite constellation, however, is limited in guaranteed response time to detect objects at GEO because it takes many hours to search the volume of space. In addition, there can be one to three orbits of latency before a satellite has access to a ground station to report its findings. Further delays can occur because the location of the sun can inhibit detection and tracking for extended periods (e.g. when the sun is behind the object of interest. Even using several satellites in LEO does not solve the viewing problem as all the LEO satellites view an object at GEO at roughly the same geometry. In addition, this planned system is also limited in its ability to detect very small objects at GEO and has no imaging capability.
Thus, conventional space situational awareness techniques have shortcomings.